As you may have noticed, the problem with running a website is that various automated bots constantly try to submit forms, press buttons, etc. Sometimes these are crawlers, but many are malicious, automated crawlers posting spammy comments, creating fake accounts, etc.
The solution to this is CAPTCHA - Completely Automated Public Turing test to tell Computers and Humans Apart - that presents a challenge to the user to check that they're human. On the submission of a form, a CAPTCHA box pops up presenting a visual challenge (with an audio version for accessibility) that has to be completed before the form is submitted.
Anubis provides that CAPTCHA functionality, and consists of two parts: an embeddable pop-up for your web page, and a server-side API for validating the result.
Why the server-side part? It's sadly essential, as any sort of interruption of a form post that can be achieved on the browser alone can be circumvented by an automated bot. It's only by performing a final validation on the server - somewhere a bot can't reach - that we can tell it's a submission from a genuine human passing the test.
That said, if you're not happy with server-side code, Anubis has a no-code, serverless option you can use in most situations.